1. Field of the Invention
The present invention relates to an ink jet recording sheet for recording with a water-color ink and a process for its production. Particularly, it relates to an ink jet recording sheet which has high glossiness comparable to commercially available cast coated paper or art coated paper and which is excellent in ink absorptivity and capable of presenting a high printed image density, and a process for its production.
2. Discussion of Background
An ink jet recording system is a system whereby fine droplets of ink are jetted and deposited on a recording sheet such as a paper sheet to record images or letters by various operational principles, and it has features such as high speed and no noise and such that multicoloring is easy, flexibility for various recording patterns is high, and no development or fixing is required. Such an ink jet recording system has been used for various applications as a recording apparatus for various patterns including Chinese characters and for color images. Further, with respect to an image formed by a multi-color ink jet system, it is possible to obtain a record comparable to a printed image by a multi-color photographic system by plate-making system. Further, in a case where the number of copies is relatively small, the ink jet recording system is inexpensive as compared with the photographic system, and it is accordingly widely applied even to the full color image recording field.
The recording sheet to be used for such an ink jet recording system is required to satisfy requirements that the printed dot density must be high, the color must be bright and clear, absorption of the ink must be swift so that even if printed dots are overlaid one on another, the ink will not run or blot, diffusion of the printed dots in a transverse direction must not be more than necessary, and the peripheries of the printed dots must be smooth and must not be blurred.
To satisfy such requirements, some proposals have been made heretofore. For example, Japanese Unexamined Patent Publication No. 53012/1977 discloses an ink jet recording sheet prepared by wetting base paper having a low sizing degree with a coating material for surface treatment. Further, Japanese Unexamined Patent Publication No. 5830/1980 discloses an ink jet recording sheet having an ink-absorbing coating layer formed on the surface of a support. Japanese Unexamined Patent Publications No. 51583/1980 and No. 157/1981 disclose embodiments in which non-glue silica powder is used as a pigment in a coating layer. Further, Japanese Unexamined Patent Publication No. 11829/1980 discloses an embodiment of coated paper of a double layered structure differing in the ink absorbing rate.
In an ink jet recording sheet, it is common to provide an ink-receiving layer employing a porous pigment and having an ink absorbing property to control the color effect and the definition which are decisive for the image quality and thereby to improve color reproducibility and image reproducibility. The ink-receiving layer having ink absorptivity is required to have many voids in the ink-receiving layer to absorb and maintain ink. However, the ink-receiving layer having many voids has difficulties that incident light to the ink-receiving layer is likely to be scattered and its transmittance tends to be prevented, whereby the ink-receiving layer tends to be opaque, and it tends to be difficult for light to reach the ink penetrated into voids, whereby the image tends to be whitened, and the color reproducibility and the color density tend to be low. The ink-receiving layer having many voids tends to have a porous surface, whereby high gloss can hardly be expected.
With respect to an ink jet recording sheet having high gloss, for example, Japanese Unexamined Patent Publication No. 197285/1986 proposes a method wherein a porous ink-receiving layer is formed on a transparent support, so that an image formed on the ink-receiving layer can be observed from the support side. Japanese Unexamined Patent Publication No. 215081/1991 proposes a method wherein a dye adsorbing layer composed of a porous alumina hydrate and a solvent absorbing layer composed of a porous fine powdery silica are sequentially laminated on a transparent substrate, so that an image formed on the dye adsorbing layer can be observed from the support side. However, these methods have drawbacks that in printing the image, it is necessary to conduct image treatment to obtain a mirror image, and the support to be used is limited to the one having transparency.
Japanese Unexamined Patent Publication No. 113986/1990 discloses a method of treating with an aqueous solution containing a cationic polymer electrolyte, followed by casting, and Japanese Unexamined Patent Publication No. 274587/1990 proposes a method wherein using colloidal silica for the improvement of gloss, treatment with an aqueous solution containing a cationic polymer electrolyte, is followed by casting. However, use of a cationic polymer electrolyte has a drawback that the cationic polymer electrolyte present on the surface when printed, will dissolve again in the ink, whereby the surface contour at the printed portion is roughened, whereby the gloss or the definition of the image at the printed portion tends to deteriorate.
Recording sheets or films are available wherein a resin capable of absorbing ink by dissolution and swelling, is coated for the purpose of imparting gloss. However, such recording sheets or films of the type to let the ink be absorbed by dissolution and swelling of the resin have problems that absorption and drying of the ink are slow, and stains or smudges are likely to result due to ink transfer, although gloss can be obtained.
For the treatment to impart gloss, it is common to employ a method wherein by means of a calender apparatus such as super calender or gloss calender, a coated sheet is passed between rolls to which a temperature and a pressure are applied, to smooth the coating layer surface. However, if calender treatment is carried out under a high linear load for the purpose of imparting gloss, although the gloss will be improved, voids in the coating layer will decrease, whereby there will be problems that absorption of ink tends to be slow, and due to inadequate absorption capacity, ink is likely to overflow. Accordingly, for the calender treatment, conditions have to be selected within a range permitted by the ink absorption capacity, and with the presently available techniques, it is difficult to attain gloss and absorption of ink simultaneously.
On the other hand, in recent years, ink jet recording sheets have found their application also to labels and tags. In the process for preparing such labels or tags, the sheets are bent or folded. Accordingly, they are required to have not only the surface strength but also flexural strength. Further, an ink jet recording apparatus is used in a manner similar to a widely used copying machine, and after copying, the recording sheets are likely to be filed or bound into a book, whereby they are required to have adequate folding strength. If the folding strength is inadequate, the folded portion tends to peel and loses the ink jet recording properties. Thus, it has become necessary to secure not only the coat strength but also the folding strength also for ink jet recording sheets.
Further, the ink jet recording system provides good definition and color effect at a level of personal computers, and it has been made possible to quickly obtain even a complicated image relatively simply. However, conventional ink jet recording sheets are poor in the gloss of sheet as compared with printing paper or photographic paper, whereby it has been difficult to use them in the field where gloss is desired e.g. in the field of posters or stickers, in view of their poor gloss. However, there is an increasing demand for recording media whereby convenience of the ink jet recording system can be utilized by overcoming the mutually opposing relation of the ink absorptivity and the gloss.